1. Field
This disclosure relates generally to integrated circuits, and more specifically, to level shifters.
2. Related Art
Level shifters are generally for changing the voltage levels of logic signals. One situation where they are commonly used is for programming and erasing of non-volatile memory (NVM) in which the logic, which determines which memory cells are to be programmed or erased, is operated at a much lower voltage than that required for performing the program or erase. For example, it is common for the logic of an integrated circuit to be operated at around one volt while the program or erase function is carried out at about ten volts. Conventional level shifters can generally be designed to be effective in this situation if the program or erase voltage is held within a narrow range. A technique for improving the endurance or reliability of NVM cells is to change the program or erase voltage based on the condition of the NVM. This can be based on the number of program and erase cycles or on the actual measured program or erase condition of a particular NVM cell. The program or erase voltage can be as low as 6 volts or as high as 14 volts. A 6 volt to 14 volt range raises some problems with conventional level shifters. One problem is the lower current drive capability at the lower voltage and the breakdown voltage of the transistors that are used in the level shifter at the higher voltage. Current drive can be increased by increasing the size of the transistors of the level shifter, but this is an undesirable solution because that increases the size of the circuit. Breakdown voltage can be addressed by adding transistors, but that not only increases size but also increases the number of transistors in the output path which increases output resistance and thus reduces speed of operation. Stacking transistors also increases the minimum operating voltage.
Thus there is a need for a level shifter that reduces or eliminates the adverse affects described above.